Various circuit topologies have been developed to control the flow of power and/or achieve efficient energy utilization in associated equipment. These technologies can generally be categorized as power electronics. The importance of power electronics in electrical equipment stems from its broad range of applications, including residential, industrial, commercial and communications devices to name a few.
Power electronics generally utilize one or more power converters to, for example, control and shape an input electrical signal into another electrical signal having different electrical characteristics, such as magnitude, frequency and/or the number of phases. With the continued advances in fabrication technologies, an increasing number of applications are making use of power converters, including power supplies as well as other power electronic converters or conditioners.
In some power converter applications, it is necessary to limit the maximum duty cycle of a signal generated by an associated pulse width modulator. For example, the pulse width modulator output signal is limited to a certain maximum duty cycle to avoid damage or failure of the power converter under various conditions, including transient and steady state conditions. The duty cycle controls the “on time” of the power converter. The possibility for damage of the power converter is due at least in part to the transformer's inability to demagnetize at large duty cycle ratios. That is, if the duty cycle becomes too large, the magnetic core can saturate and thereby cause permanent damage to the power converter circuitry. Similar concerns exist for other inductive applications. Accordingly, it becomes increasingly important to set a maximum duty cycle accurately. Thus, it becomes significant to limit the current in the inductor to avoid saturation. The flux in the inductor windings is proportion to the applied DC voltage and time, which electrical characteristics can be limited by a corresponding volt-second clamp.
Various approaches exist to limit a maximum duty cycle, typically including circuitry operative to clamp the output signal of the pulse width modulator. Operation of clamping circuitry can vary according to several factors, including, for example, accuracy and temperature stability of the clamp circuitry and associated reference voltages. The cumulative effect of such variations can amount to a sizeable tolerance in the maximum duty cycle setting.
One conventional approach for limiting maximum duty cycle is to average and compare the output signal of a pulse width modulation to a reference value. The reference value is proportional to the peak value of the comparator's output voltage. A closed loop circuit continually adjusts the duty cycle so the difference between the reference value and the average comparator output is substantially mitigated, which provides a maximum duty cycle output. Other approaches exist, but generally are unable to achieve a desired level of accuracy for either a maximum duty cycle clamp or volt-second clamp.